The separation of fine particles of coal contained in a coal slurry through the use of froth flotation processes is well known. Froth flotation processes involve introducing air into the coal slurry. The hydrophobic particles of coal are contacted with finely disseminated air bubbles such that the fine air bubbles become adhered to the hydrophobic coal particles. The surface tension of the air bubble is such that small particulates, typically those less than a particle size of 28 mesh X 0, readily attach themselves. The particle carrying bubbles are then permitted to rise, forming a froth on the surface of the slurry. The froth, containing the hydrophobic particles of coal, is skimmed from the surface of the slurry and collected, while rejecting any hydrophilic particles of impurities which do not adhere to the air bubbles and which remain suspended in the slurry. These processes are generally described in the texts An Introduction to the Theory of Flotation, V. I. Klassen and V. A. Mokrousov, Butterworths, 1963, and Froth Flotation, 50th Anniversary Volume, D. Furstenau, AIME, 1962.
Flotation of coal fines has become increasingly important as a separation and cleaning process where there is a lowering in both the particle size and grade of the coal being recovered from mining operations. The ability to remove the coal fines from coal washery waters or tailings is also advantageous in order to recover coal fines missed by other techniques of coal recovery.
In order to improve the selectivity and recovery of the flotation process and enhance floating of the coal fines, various types of reagents have been developed for addition to the slurry. Frothers and collectors are two types of reagents which are commonly used in coal flotation.
The purpose of a frother is to facilitate the production of a more stable froth which is better able to carry the particles of coal on the surface of the slurry until the froth is removed. Stability is improved because the frother enhances the attachment of the air bubble to the coal particles. Most high rank coals are naturally floatable due to the hydrophobic nature of their surfaces, which causes them to be attracted to the air bubbles. Therefore flotation of high rank coals may generally be effected with the use of a conventional frother alone. However, oxidized bituminous and low rank coals tend to be more hydrophilic in nature and therefore are difficult or impossible to float, because the coal particles are less attracted to the air bubbles. Attempts have been made to develop frothers more suited for flotation of these types of coals. For example, U.S. Pat. No. 4,504,385 issued Mar. 12, 1985 to Keys is directed towards an improved alcohol frother, and U.S. Pat. No. 4,308,133 issued Dec. 29, 1981 to Meyer is directed towards a froth promoter which is added contemporaneously to the slurry with the frother in order to enhance the formation of the froth on the surface of the slurry.
Collectors are used in conjunction with frothers and are intended to aid in floating those coals which are less hydrophobic in nature and therefore less readily floated. The basic purpose of a collector is to render the surfaces of the particles of coal more hydrophobic such that the particles of coal and the rising air bubbles which are coated with the frother have greater contact and adhesion. The collector is generally selective in that it selectively adheres to and preferentially wets the surfaces of the particles of coal but not the particles of impurities and other matter contained in the slurry. Collectors are usually a hydrocarbon oil. Diesel fuel, fuel oil and kerosene are the most widely used. Attempts have been made to improve the effectiveness of the collector. Examples of patents directed at improved collectors include U.S. Pat. No. 4,416,769 issued Nov. 22, 1983 to McCaffrey et. al., U.S. Pat. No. 4,526,680 issued Jul. 2, 1985 to Owen, and U.S. Pat. No. 4,532,032 issued Jul. 30, 1985 to Ng et. al.
Despite the use of frothers and collectors, as coal becomes more oxidized or of a lower rank, it becomes more hydrophilic and less easy to float. As a result, if a collector or frother is utilized with oxidized or low rank coals, relatively large quantities are required to float the particles of coal and flotation is not optimum.
To improve the flotation of particles of coal which have a more hydrophilic nature, other types of reagents have been developed which are usually used in conjunction with collectors and frothers. U.S. Pat. No. 4,589,980 issued May 20, 1986 to Keys, and U.S. Pat. No. 4,678,561 and 4,678,562 issued Jul. 7, 1987 to Keys are directed at the addition of a reagent, referred to as a "promoter", to the slurry along with a collector and a frother. The promoter is comprised of a non-ionic, hydrophobic, non-emulsified, aliphatic ester of an at least 10 aliphatic carboxylic acid which is devoid of nitrogen and sulphur atoms or the carboxylic acid itself. Once all of the reagents are added, the slurry is conditioned by vigorously mixing or agitating the slurry prior to flotation.
Similarly, other processes combine the collector and the frother with other reagents to form a product which is then added to the slurry and dispersed into the slurry in a single agitation or mixing process step. Examples include U.S. Pat. No. 4,632,750 issued Dec. 30, 1986 to McGarry, U.S. Pat. No. 4,857,221 issued Aug. 15, 1989 to Brookes et. al., U.S. Pat. No. 4,305,815 issued Dec. 15, 1981 to Hefner, Jr., U.S. Pat. No. 4,308,132 issued Dec. 29, 1981 to McCarthy, U.S. Pat. No. 4,372,864 issued Feb. 8, 1983 to McCarthy, U.S. Pat. No. 4,452,714 issued Jun. 5, 1984 to McCarthy, and U.S. Pat. No. 4,474,619 issued Oct. 2, 1984 to Meyer et. al.
The processes which have been developed tend not to be very selective, are uneconomical, and are therefore not widely used. There is therefore a need in the industry for a process for floating particles of oxidized bituminous and low ranked coals contained in a coal slurry in an economical manner using conventional coal flotation techniques.